Not necessarily. But storing them in plain text on the server and all history accessible just by hijacking a phone number (or a single text message) is, considering the alternatives, don't you agree?
Because there is no end to end encryption (other than some very limited temporary one on one chats that only work between two mobile devices) which means the server can (and does) read and store all your messages. Getting access to your full chat history is as simple as intercepting a single text, because the server has access to your plain text messages.
They might do some form of encryption to store the messages, but this is meaningless when they also have access to the keys and can decrypt any message whenever they want.